CN111502803B

CN111502803B - Aftertreatment system and aftertreatment method for lean burn engine

Info

Publication number: CN111502803B
Application number: CN201910244967.7A
Authority: CN
Inventors: 朱南鲁; 金昌焕; 郑昌镐; 刘哲豪
Original assignee: Hyundai Motor Co; Kia Motors Corp
Current assignee: Hyundai Motor Co; Kia Corp
Priority date: 2019-01-31
Filing date: 2019-03-28
Publication date: 2022-03-25
Anticipated expiration: 2039-03-28
Also published as: CN111502803A; US10697340B1; DE102019208436A1; KR20200095295A

Abstract

The invention discloses an aftertreatment system and an aftertreatment method for a lean burn engine. The aftertreatment method is configured to control an aftertreatment system equipped with an ammonia generating catalyst module, a selective catalytic reduction catalyst, and a CO purification catalyst in this order on an exhaust pipe through which exhaust gas flows. In the aftertreatment method, the engine is operated at a stoichiometric air/fuel ratio (AFR) and a lean AFR in sequence before entering the rich AFR.

Description

Aftertreatment system and aftertreatment method for lean burn engine

Technical Field

The invention relates to an aftertreatment system and an aftertreatment method for a lean burn engine.

Background

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

The vehicle may be provided with at least one catalytic converter for reducing Emissions (EM) contained in the exhaust gas. Exhaust gas flowing out of the engine through an exhaust manifold (exhaust manifold) is pushed into a catalytic converter mounted on an exhaust pipe and purified therein. After that, the noise of the exhaust gas is reduced while flowing through the muffler, and then the exhaust gas is discharged to the air through the tail pipe. The catalytic converter purifies EM contained in the exhaust gas. In addition, a particulate filter for trapping Particulate Matter (PM) in the exhaust gas is installed in the exhaust pipe.

A three-way catalyst (three-way catalyst, TWC) is a catalytic converter, and reacts with Hydrocarbon (HC) compounds, carbon monoxide (CO), and nitrogen oxides (NOx) as harmful components in exhaust gas to remove these compounds. The TWC is mainly mounted in gasoline vehicles, and Pt/Rh, Pd/Rh, or Pt/Pd/Rh systems are used as the TWC.

Lean-burn engines of gasoline engines improve fuel efficiency by combusting a lean air/fuel mixture. Lean-burn engines combust a lean air/fuel mixture, so the air/fuel ratio (AFR) of the exhaust is also lean. However, when the AFR is lean, the TWC may slip NOx out (slip, escape, slip) without adequately reducing all of the NOx contained in the exhaust. Accordingly, vehicles equipped with lean-burn engines may include a Selective Catalytic Reduction (SCR) catalyst for purifying NOx flowing through the TWC. The SCR catalyst used in a vehicle equipped with a lean burn engine may be a passive type SCR catalyst.

When the AFR is rich, the TWC may reduce NOx to produce NH₃And NH generated in the TWC₃Stored in a passive SCR catalyst. The passive SCR catalyst uses the stored NH when the AFR is lean₃NOx contained in exhaust gas is purified.

Lean-burn engines equipped with a TWC and a passive SCR catalyst may adjust the AFR to a richer state by adding fuel for a predetermined duration to bring a sufficient amount of NH₃Stored in a passive SCR catalyst. If the amount of NOx emitted from a lean-burn engine increases, the number and duration of lean-burn engine operations at a rich AFR also increases. Therefore, a decrease in fuel economy may occur.

To mitigate the degradation of fuel economy, the NH generated at the rich AFR must be increased₃The amount of (c). In this case, the duration of the hold at the rich AFR may be reduced, thereby suppressing deterioration of fuel economy. An Ammonia Producing Catalyst (APC) may be added downstream of the TWC to increase NH produced at the rich AFR₃The amount of (c). The APC may store NOx contained in the exhaust at a lean AFR and generate NH from the stored NOx and NOx contained in the exhaust at a rich AFR₃. Thus, when under a rich AFR, APC can produce more NH than TWC₃。

However, since the APC includes a component capable of storing NOx, if the engine is operated at a rich AFR with the temperature of the APC being low, nitrous oxide (N) may be generated in the APC₂O). Thus, it has been found that if the temperature of the APC is low when a transition to the rich AFR is desired or required, the APC should be heated to a predetermined temperature.

Furthermore, it has been found that CO and HC can escape from the TWC at rich AFR. CO and HC that escape from the TWC may not be purified, but discharged to the outside of the vehicle. Therefore, it may be desirable to reduce the amount of NH generated in order to produce NH₃And an additional catalytic converter or control for setting the AFR to rich with the CO and HC evolved.

The above information disclosed in this background of the invention section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.

Disclosure of Invention

The present disclosure has been made in an effort to provide an aftertreatment system for a lean burn engine, which is advantageous in that it can reduce the amount of carbon monoxide emitted to the outside of a vehicle at a rich AFR while improving ammonia production.

Another aspect of the present disclosure provides an aftertreatment method for a lean burn engine that is further advantageous in that nitrous oxide and carbon monoxide that may be emitted to the outside of a vehicle can be reduced by effectively heating an ammonia-generating catalyst in cases where a shift to a rich AFR is desired or necessary.

According to the disclosureThe aftertreatment system for a lean burn engine of the opening aspect may include: an exhaust pipe that is connected to a lean burn engine and through which exhaust gas generated in the lean burn engine flows; a three-way catalyst (TWC) that is mounted on an exhaust pipe and purifies Hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) contained in exhaust gas; an Ammonia Producing Catalyst (APC) mounted on the exhaust pipe downstream of the TWC, the ammonia producing catalyst storing NOx at a lean air/fuel ratio (AFR) and generating H at a rich AFR₂Releasing stored NOx, and using released NOx and generated H₂Generation of ammonia (NH)₃) (ii) a A Selective Catalytic Reduction (SCR) catalyst installed on the exhaust pipe downstream of the APC to store NH generated in the APC₃And passing the stored NH₃Reducing NOx contained in the exhaust gas; a CO purification catalyst (CO removal catalyst, CO clean-up catalyst, CUC) that is installed on the exhaust pipe downstream of the SCR catalyst and purifies CO contained in the exhaust gas; and a controller that detects information about the AFR and the temperature of the exhaust gas and controls the AFR of the exhaust gas based on the information about the AFR and the temperature of the exhaust gas, wherein the controller compares the temperature of the APC to a threshold temperature in response to detecting that a transition to the rich AFR is desired, and operates the engine at a stoichiometric AFR before the transition to the rich AFR when the temperature of the APC is below the threshold temperature.

The controller may operate the engine at the stoichiometric AFR for a first predetermined duration.

The first predetermined duration may be determined based on the temperature of the APC at a detection time when a transition to the rich AFR is necessary or desired.

After operating the engine at the stoichiometric AFR, the controller may operate the engine at the target lean AFR for a second predetermined duration before transitioning to the rich AFR.

The second predetermined duration may be determined based on the first predetermined duration, the target lean AFR, and the temperature of the CUC.

After operating the engine at the target lean AFR for the second predetermined duration, the controller may operate the engine at the target rich AFR for a rich duration.

The rich duration may be determined based on the target rich AFR and the temperature of the CUC.

The rich duration may be calculated such that if the engine is operated at the target rich AFR for the rich duration, the escape of CO accumulated downstream of the CUC during the rich duration is less than or equal to a predetermined value.

The aftertreatment system may further include a particulate filter disposed between the TWC and the APC or the APC and the SCR catalyst, wherein the particulate filter traps particulate matter in the exhaust.

An aftertreatment method according to another aspect of the disclosure is configured to control an aftertreatment system equipped with a three-way catalyst (TWC), an ammonia generating catalyst (APC), a Selective Catalytic Reduction (SCR) catalyst, and a CO purification catalyst in that order on an exhaust pipe through which exhaust gas flows and which is connected to a lean-burn engine.

The post-processing method can comprise the following steps: operating the engine at a lean AFR; calculating NH stored in SCR catalyst₃The amount of (c); determining whether a transition to a rich AFR is necessary or desired; in the event that a transition to rich AFR is necessary or desirable, determining whether the temperature of the APC is greater than or equal to a threshold temperature; operating the engine at stoichiometric AFR for a first predetermined duration when the temperature of the APC is below a threshold temperature; and operating the engine at the target rich AFR for the rich duration.

The first predetermined duration may be determined based on the temperature of the APC at a determined time when a transition to the rich AFR is desired.

The post-processing method may further comprise: after operating the engine at the stoichiometric AFR for the first predetermined duration and before operating the engine at the rich AFR for the rich duration, operating the engine at the target lean AFR for a second predetermined duration.

In one aspect, the determination of whether a transition to rich AFR is desired may include calculating an amount of NOx to flow into the SCR catalyst, wherein NH is stored in the SCR catalyst₃Is less than the amount of NH required to purify the NOx that will flow into the SCR catalyst₃May determine that a transition to a rich AFR is desired.

In another aspect, the determination of whether a transition to rich AFR is desired may include storing NH in the SCR catalyst₃With a predetermined NH ratio₃Comparing the lower threshold value with the NH stored in the SCR catalyst₃Is less than a predetermined NH₃A lower threshold, it may be determined that a transition to rich AFR is desired.

According to aspects of the present disclosure, an APC may be disposed between the TWC and the SCR catalyst to increase NH supplied to the SCR catalyst at the rich AFR₃The amount of (c). Thus, the duration and number of times the engine is operated at the rich AFR may be reduced, thereby improving fuel economy.

Further, the CUC may be disposed downstream of the SCR catalyst to purify CO escaping from the TWC as well as the APC.

Further, the APC may be warmed by operating the engine at stoichiometric AFR prior to transitioning to rich AFR. Thus, nitrous oxide generation at a concentrated AFR may be reduced.

Further, if the engine is operating at stoichiometric AFR, the engine is again operated at lean AFR to obtain the oxygen storage capacity of the CUC, and then operated at rich AFR. Therefore, the decrease in the CO purification performance can be suppressed or prevented.

In addition, other effects of aspects of the present invention should be described directly or implicitly in the description provided herein. Various effects predicted according to aspects of the present invention will be disclosed in the description provided herein.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

Drawings

In order that the invention may be well understood, various forms thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an aftertreatment system for a lean burn engine, according to aspects of the present disclosure;

FIG. 2 is a schematic diagram illustrating an aftertreatment system for a lean burn engine according to another aspect of the disclosure;

FIG. 3 is a schematic diagram illustrating an aftertreatment system for a lean burn engine according to other aspects of the disclosure;

FIG. 4 is a block diagram illustrating an aftertreatment system for a lean burn engine, according to aspects of the present disclosure;

FIG. 5 is a flow chart illustrating a post-processing method according to aspects of the present disclosure;

FIG. 6 is a graph illustrating the temperature of the TWC, the temperature of the APC, and the concentration of stored NOx escaping from the APC when the engine is operated sequentially at lean AFR, stoichiometric AFR, and lean AFR;

FIG. 7 is a graph illustrating the concentration of stored NOx escaping from the APC and the maximum concentration of nitrous oxide generation as a function of the temperature of the APC upon entering the rich AFR;

FIG. 8 is a graph showing the amount of fuel used to heat the APC and the maximum concentration of nitrous oxide generation entering rich AFR, the APC being heated without heating the APC, the APC being heated by operating the engine only at stoichiometric AFR, and the APC being heated by operating the engine sequentially at stoichiometric AFR and lean AFR, respectively; and

FIG. 9 is a graph showing the accumulated amount of CO slip from the CUC and the maximum concentration of nitrous oxide generation over a predetermined duration for a rich AFR, entering the rich AFR where the engine is warmed without warming the APC, where the APC is warmed by operating the engine only at the stoichiometric AFR, and where the APC is warmed by operating the engine sequentially at the stoichiometric AFR and the lean AFR, respectively.

It should be understood that the above-described drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the invention, including, for example, specific dimensions, orientations, locations, and shapes, will be determined in part by the particular intended application and use environment.

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

Detailed Description

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. The term "coupled" refers to a physical relationship between two components in which the components are either directly connected to one another or indirectly connected through one or more intermediate components.

It should be understood that the terms "vehicle," "vehicular," "automobile," or other similar terms as used herein include motor vehicles, typically, passenger automobiles including, for example, Sport Utility Vehicles (SUVs), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and include hybrid vehicles, electric vehicles, hybrid electric vehicles, hydrogen-powered vehicles, and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum).

Additionally, it should be understood that one or more of the following methods or aspects thereof may be performed by at least one controller. The term "controller" may refer to a hardware device that includes a memory and a processor. The memory is configured to store program instructions, and the processor is specifically programmed to execute the program instructions to perform one or more processes described further below. Moreover, it should be understood that the following methods may be performed by a system including a controller, as described in detail below.

Further, the controller of the present invention may be embodied as a non-transitory computer readable medium containing executable program instructions executed by a processor or the like. Examples of computer readable media include, but are not limited to, ROM, RAM, Compact Disc (CD) -ROM, magnetic tape, floppy disk, flash drive, smart card, and optical data storage. The computer readable recording medium CAN also be distributed throughout a computer network so that program instructions are stored and executed in a distributed fashion, such as through a telematics server or a Controller Area Network (CAN).

Hereinafter, aspects of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic illustration of an aftertreatment system for a lean burn engine according to one aspect of the invention.

As shown in FIG. 1, an aftertreatment system according to one aspect of the invention includes an engine 10, an exhaust pipe 20, an ammonia producing catalyst module 35, a Selective Catalytic Reduction (SCR) catalyst 50, and a CO clean-up catalyst (CUC) 60.

Engine 10 combusts an air/fuel mixture to convert chemical energy into mechanical energy. Engine 10 is coupled to an intake manifold 16 to flow air into combustion chambers 12. Exhaust gas generated during the combustion process is collected in an exhaust manifold 18 and then flows out of the engine 10. The combustion chamber 12 is equipped with a spark plug 14 to ignite the air/fuel mixture within the combustion chamber 12. The engine 10 may be a gasoline engine. Depending on the type of gasoline engine, fuel may be directly injected into combustion chamber 12, or an air/fuel mixture may be supplied to combustion chamber 12 via intake manifold 16. Additionally, engine 10 may be a lean burn engine. Therefore, the engine 10 is operated at a lean air/fuel ratio (AFR) except for a specific driving condition.

An exhaust pipe 20 is connected to the exhaust manifold 18 to discharge exhaust gas to the outside of the vehicle. The exhaust pipe 20 is equipped with an ammonia generating catalyst module 35, an SCR catalyst 50, and a CUC60 to purify or remove emissions contained in the exhaust gas.

The ammonia producing catalyst module 35 includes a three-way catalyst (TWC)30 and an Ammonia Producing Catalyst (APC) 40. The TWC 30 and APC40 may be disposed in one housing, but are not limited thereto. The ammonia producing catalyst module 35 may generate NH at the rich AFR from NOx contained in the exhaust or stored in the ammonia producing catalyst module 35₃. The ammonia producing catalyst module 35 includes an oxygen storage material having an Oxygen Storage Capacity (OSC).

The TWC 30 is provided on the exhaust pipe 20 through which exhaust gas discharged from the engine 10 flows, and harmful materials (including CO, HC, and NOx) contained in the exhaust gas are converted into harmless components by an oxidation-reduction reaction in the TWC 30. In particular, the TWC 30 may reduce NOx contained in the exhaust to NH at the rich AFR₃. At this time, the TWC 30 may not sufficiently purify CO and HC in the exhaust gas, and CO and HC may escape therefrom. Further, the TWC 30 oxidizes CO and HC contained in the exhaust gas at a lean AFR. Since the TWC 30 is well known to those of ordinary skill in the art, a detailed description thereof will not be provided.

The APC40 is disposed on the exhaust pipe 20 downstream of the TWC 30. The APC40 stores NOx contained in the exhaust at a lean AFR and generates H at a rich AFR₂To release the stored NOx and then use the released NOx with the generated H₂Formation of NH₃。

In one aspect, the APC40 comprises 0.4 to 0.9 wt% Pt, 0.057 to 0.3 wt% Pd, 0.03 to 0.1 wt% Rh, 5.0 to 15.0 wt% Ba, 10 to 30 wt% CeO, based on the total weight of the APC40₂48.7-84.513 wt% of MgO-Al₂O₃A compound, and 0-5 wt% of an additive.

In another aspect, the APC40 comprises 0.4 to 0.9 wt% Pt, 0.057 to 0.3 wt% Pd, 0.03 to 0.1 wt% Pd, based on the total weight of the APCRh, Ba 5.0-15.0 wt%, CeO 10-25 wt%₂48.7-79.513 wt% of MgO-Al₂O₃A compound, and 0-10 wt% of an additive.

The additive is added to improve CeO₂And Al₂O₃And it comprises at least one of La, Zr, Mg and Pr.

The function of Pt contained in the APC40 is to oxidize NOx, thereby enabling the APC40 to store NOx. Furthermore, Pt increases H generated in APC40₂The amount of (c).

The Pd contained in the APC40 improves the heat resistance of the APC 40. Since the APC40 is disposed adjacent to the engine 10, the temperature of the APC40 may rise to 950 ℃. Therefore, Pd is added to the APC40 to improve the heat resistance.

To increase NH₃Generation and H₂The weight ratio of Pt to Pd in the APC40 can be 3:1 to 7: 1. The weight ratio of Pt to Pd in the APC40 can be 3:1 to 5: 1.

The Rh contained in the APC40 purifies NOx contained in the exhaust gas at the stoichiometric AFR.

Ba and CeO contained in APC40₂Configured to store NOx in the form of nitrates. CeO (CeO)₂Containing an oxygen storage material.

Further, CeO₂Increase of H₂And (4) generating. However, if the APC40 contains a large amount of CeO₂Then NH is generated₃It may be oxidized again. Thus, the APC40 may comprise 10-30 wt.% CeO, based on the total weight of the APC40₂。

MgO-Al contained in APC40₂O₃The composite is used as a substrate. With MgO-Al₂O₃MgO-Al based on the total weight of the composite₂O₃The composite may include 15-25 wt% MgO. MgO enhances the thermal stability of Ba.

An SCR catalyst 50 is mounted on the exhaust pipe 20 downstream of the APC 40. The SCR catalyst 50 stores NH generated in the ammonia producing catalyst module 35 (i.e., the TWC 30 and the APC 40) at the rich AFR₃And passing the stored NH under lean AFR₃Reduction of N contained in exhaust gasOx. This type of SCR catalyst 50 may preferably be a passive SCR catalyst 50.

The SCR catalyst 50 may be made of porous Al supported on₂O₃One or a combination of zeolite catalyst and metal catalyst in (1). At least one of Cu, Pt, Mn, Fe, Co, Ni, Zn, Cs and Ga may be ion exchanged in the zeolite catalyst. In porous Al supported on₂O₃In the metal catalyst of (1), at least one metal selected from Pt, Pd, Rh, Ir, Ru, W, Cr, Mn, Fe, Co, Cu, Zn and Ag may be supported on porous Al₂O₃In (1).

The CUC60 is mounted on the exhaust pipe 20 downstream of the SCR catalyst 50. The CUC60 purifies CO contained in exhaust gas. In particular, CO may escape from the ammonia producing catalyst module 35 (i.e., the TWC 30 and the APC 40) at a rich AFR. Therefore, by placing CUC60 most downstream of the aftertreatment system, emission of CO to the outside of the vehicle can be suppressed or prevented. The CUC60 is loaded in CeO₂And Al₂O₃Pt, Pd, Rh and Ba in (1).

In one aspect, CUC60 comprises 0.2-1.5 wt% Pt, 0-0.4 wt% Pd, 0-0.4 wt% Rh, 0-5.0 wt% Ba, 40-90 wt% CeO, based on the total weight of CUC60₂9.8-59.8 wt% of Al₂O₃And 0-10 wt% of additives.

In another aspect, CUC60 includes 0.2-1.5 wt% Pt, 0-0.4 wt% Pd, 0-0.4 wt% Rh, 0-5.0 wt% Ba, 40-90 wt% CeO, based on the total weight of CUC60₂9.8-59.8 wt% of Al₂O₃And 0-20 wt% of additives.

Additives are added to improve CeO₂And Al₂O₃And comprises at least one of La, Zr, Mg and Pr.

CUC60 is mainly composed of Pt-CeO₂And (4) forming. Herein, Pt is used for oxidizing CO, and CeO₂Oxygen storage materials with Oxygen Storage Capacity (OSC) are included to aid in the oxidation of CO at low temperatures under dilute AFR. Pd/Al₂O₃Also plays a role in combination with Pt/CeO₂Similar action, but Pt/CeO₂May be greater than Pd/Al₂O₃In order to improve the oxidizing power at low temperatures.

Ba contained in CUC60 is used to remove a small amount of NOx not removed from the SCR catalyst 50 when AFR is rich.

The inclusion of Rh in the CUC60 is intended to promote the reduction of NOx when the AFR is rich.

The exhaust pipe 20 may be equipped with a plurality of

sensors

32, 34, 36, 62, and 64 for detecting the AFR of the exhaust gas and the operation of the

catalysts

30, 40, 50, and 60.

A first oxygen sensor 32 is mounted on the exhaust pipe 20 upstream of the TWC 30, and detects O in the exhaust gas upstream of the TWC 30₂The concentration, and a signal corresponding thereto is transmitted to the controller 90. The AFR of the exhaust gas described herein (hereinafter referred to as 'λ') may refer to a value detected by the first oxygen sensor 32. Additionally, the AFR control described herein may refer to controlling the AFR of the exhaust to a target AFR.

A second oxygen sensor 34 is mounted on the exhaust pipe 20 downstream of the TWC 30, and detects O in the exhaust gas downstream of the TWC 30₂The concentration, and a signal corresponding thereto is transmitted to the controller 90.

A third oxygen sensor 36 is mounted on the exhaust pipe 20 downstream of the APC40, and detects O in the exhaust gas downstream of the APC40 (i.e., the ammonia producing catalyst module 35)₂Concentration and transmits a signal corresponding thereto to the controller 90. The value detected by the third oxygen sensor 36 is used to determine whether the OSC of the ammonia generating catalyst module 35 has been completely consumed.

The first temperature sensor 62 is mounted on the exhaust pipe 20 upstream of the SCR catalyst 50, detects the temperature of the exhaust gas upstream of the SCR catalyst 50, and transmits a signal corresponding thereto to the controller 90.

The second temperature sensor 64 is mounted on the exhaust pipe 20 downstream of the SCR catalyst 50, detects the temperature of the exhaust gas downstream of the SCR catalyst 50, and transmits a signal corresponding thereto to the controller 90.

In addition to the

sensors

32, 34, 36, 62, and 64 described herein, the aftertreatment system may further include various sensors. For example, additional temperature sensors may be mounted on the exhaust pipe 20 upstream and downstream of the TWC 30 to detect the temperature of the exhaust gas upstream and downstream of the TWC 30, respectively. Additionally, as shown in FIG. 4, the aftertreatment system may further include an air flow meter 66. In addition, the aftertreatment system may further include a NOx sensor, an HC sensor, or a CO sensor mounted on the exhaust pipe 20, and the concentration of the emissions contained in the exhaust gas may be detected via these sensors.

The controller 90 is electrically connected to the

sensors

32, 34, 36, 62, 64, and 66 to receive signals corresponding to the detected values through the

sensors

32, 34, 36, 62, 64, and 66, and determine the driving conditions of the vehicle, the AFR, and the temperatures of the

catalysts

30, 40, 50, and 60 based on the signals. The controller 90 may control the ignition timing, the fuel injection timing, the fuel amount, and the like by controlling the engine 10 based on the determination result. The controller 90 may be implemented with at least one processor executed by a predetermined program, and the predetermined program may be programmed to perform each step of the post-processing method according to an aspect of the present invention.

FIG. 2 is a schematic diagram illustrating an aftertreatment system for a lean burn engine according to another aspect of the disclosure. The aftertreatment system shown in fig. 2 is a variation of the aftertreatment system shown in fig. 1.

As shown in fig. 2, the aftertreatment system according to another aspect of the present disclosure is equipped with a TWC 30, a particulate filter (gasoline particulate filter (GPF))70, an APC40, an SCR catalyst 50, and a CUC60, which are provided on an exhaust pipe 20 in this order. Since the TWC 30, APC40, SCR catalyst 50, and CUC60 are described above, they will not be described in detail.

The particulate filter 70 is mounted on the exhaust pipe 20 downstream of the TWC 30, traps particulate matter contained in the exhaust gas, and burns the trapped particulate matter. The particulate filter 70 is provided with an inlet unit and an outlet unit alternately disposed in the housing, and a wall is disposed between the inlet unit and the outlet unit. The inlet unit has one end that is open and the other end that is blocked, and the outlet unit has one end that is blocked and the other end that is open. The exhaust gas flows into the particulate filter 70 through the open end of the inlet unit, flows to the outlet unit through the wall, and flows out to the outside of the particulate filter 70 through the open end of the outlet unit. When the exhaust gas passes through the wall, particles contained in the exhaust gas do not pass through the wall and remain in the inlet unit.

FIG. 3 is a schematic illustrating an aftertreatment system for a lean burn engine according to other aspects of the disclosure. The aftertreatment system shown in fig. 3 is a variation of the aftertreatment system shown in fig. 1.

As shown in fig. 3, the aftertreatment system according to other aspects of the present disclosure is equipped with a TWC 30, an APC40, a particulate filter 70, an SCR catalyst 50, and a CUC60, which are disposed on an exhaust pipe 20 in this order. Since the TWC 30, APC40, particulate filter 70, SCR catalyst 50, and CUC60 are described above, they will not be described in detail.

FIG. 4 is a block diagram of an aftertreatment system for a lean-burn engine according to one aspect of the invention.

Fig. 4 shows a simple example of the inputs and outputs of a controller 90 to implement an aftertreatment system according to an aspect of the invention. It should be understood that the inputs and outputs of the controller 90 according to aspects of the present invention are not limited to the example shown in fig. 4.

As shown in fig. 4, the controller 90 is electrically connected to the first, second, and

third oxygen sensors

32, 34, and 36, the first and

second temperature sensors

62 and 64, and the air flow meter 66, and receives signals corresponding to the values detected by the

sensors

32, 34, 36, 62, 64, and 66.

The first oxygen sensor 32 detects O contained in the exhaust gas upstream of the TWC 30₂And transmits a signal corresponding thereto to the controller 90. The second oxygen sensor 34 detects O contained in the exhaust gas downstream of the TWC 30₂And transmits a signal corresponding thereto to the controller 90. The controller 90 may determine whether the TWC 30 is operating normally based on the signals of the first and

second oxygen sensors

32, 34 and execute AFR control of the engine 10.

Further, the third oxygen sensor 36 detects O contained in the exhaust gas downstream of the ammonia producing catalyst module 35₂And transmits a signal corresponding thereto to the controlAnd a device 90. The controller 90 determines whether the OSC of the ammonia generating catalyst module 35 has been completely consumed based on the signal of the third oxygen sensor 36. For example, if the signal of the third oxygen sensor 36 indicates that the AFR downstream of the ammonia producing catalyst module 35 is rich, the controller 90 determines that the OSC of the ammonia producing catalyst module 35 has been completely consumed.

The first temperature sensor 62 detects the exhaust temperature upstream of the SCR catalyst 50 and transmits a signal corresponding thereto to the controller 90. The second temperature sensor 64 detects the exhaust gas temperature downstream of the SCR catalyst 50 and transmits a signal corresponding thereto to the controller 90. The controller 90 may calculate the temperatures of the TWC 30, APC40, SCR catalyst 50, and CUC60 based on the signals of the first and

second temperature sensors

62, 64.

An air flow meter 66 is mounted on the intake pipe or intake duct to detect the amount of air flowing into the intake system and transmit a signal corresponding thereto to the controller 90.

Controller 90 controls the operation of engine 10 based on the values detected by

sensors

32, 34, 36, 62, 64, and 66. That is, the controller 90 may adjust the fuel injection amount to adjust the target AFR, and may retard the ignition timing for warming the

catalysts

30, 40, 50, and 60. .

Hereinafter, referring to fig. 5, a post-processing method according to an aspect of the present disclosure will be described in detail.

Fig. 5 is a flow chart illustrating a post-processing method according to aspects of the present disclosure.

As shown in fig. 5, when the engine 10 is started at step S110, the controller 90 calculates the temperatures of the

catalysts

30, 40, 50, and 60. To implement the post-treatment method according to aspects of the present invention, it is necessary to activate the

catalysts

30, 40, 50 and 60. Therefore, if the

catalysts

30, 40, 50, and 60 are not activated, the controller 90 warms the

catalysts

30, 40, 50, and 60 at step S120. That is, the ignition timing is retarded or the fuel injection amount is increased to increase the temperature of the exhaust gas.

When warming of the

catalysts

30, 40, 50, and 60 is completed, the controller 90 operates the engine 10 under the lean AFR at step S130. Therefore, the TWC 30 purifies CO and HC contained in the exhaust gas, and the particulate filter 70 collects particulate matter contained in the exhaust gas. Further, the APC40 stores at least a portion of NOx contained in the exhaust gas.

The controller 90 calculates NH stored in the SCR catalyst 50 at step S140₃The amount of (c). That is, the NH stored in the SCR catalyst 50 is calculated based on the operation history of the engine 10, the temperature history of the SCR catalyst 50, and the like₃The amount of (c).

Thereafter, based on NH stored in the SCR catalyst 50 at step S150₃The controller 90 determines whether a rich AFR is desired or required, i.e., whether a transition to a rich AFR is desired.

In one aspect, to determine whether a transition to rich AFR is desired, the controller 90 calculates an amount of NOx to flow into the SCR catalyst 50. The amount of NOx generated in the engine 10 is calculated based on the combustion state of the engine 10 (e.g., combustion temperature, combustion pressure, air amount, fuel amount, etc.), and the amount of NOx that escapes from the ammonia producing catalyst module 35 is calculated based on the AFR of exhaust gas, the temperature of the TWC 30, the temperature of the APC40, etc.

Thereafter, the controller 90 determines whether the SCR catalyst 50 can purify NOx. That is, the NH stored in the SCR catalyst 50 is determined₃Whether the amount is sufficient to purify the NOx flowing into the SCR catalyst 50. For example, if NH is stored in the SCR catalyst 50₃In an amount greater than or equal to NH required to purify NOx to be flowed into the SCR catalyst 50₃The controller 90 determines that the SCR catalyst 50 can purify NOx. Conversely, if NH is stored in the SCR catalyst 50₃In an amount less than the NH required to purify the NOx to be flowed into the SCR catalyst 50₃The controller 90 determines that a transition to rich AFR is desired.

In another aspect, to determine whether a transition to rich AFR is desired, the controller 90 determines NH stored in the SCR catalyst 50₃Whether or not the amount of (A) is greater than or equal to NH₃Lower threshold (minimum). For example, if NH is stored in the SCR catalyst 50₃In an amount greater than or equal to NH₃The controller 90 determines that a transition to rich AFR is not desired. Conversely, if stored in SCR catalyst 50NH in (1)₃Is less than NH₃The controller 90 determines that a transition to rich AFR is desired.

If it is determined at step S150 that a transition to the rich AFR is desired, the controller 90 determines at step S160 whether the temperature of the APC40 is greater than or equal to a threshold temperature. For example, the controller 90 detects/calculates the temperature of the APC40 based on the detection values of the first and

second temperature sensors

62 and 64 and/or the detection value of an additional temperature sensor mounted on the exhaust pipe 20, and the detected/calculated temperature of the APC40 is higher than or equal to a predetermined threshold temperature. In one aspect, the threshold temperature may be greater than or equal to 410 ℃ and less than or equal to 430 ℃. In another aspect, the threshold temperature may be 420 ℃.

If the temperature of the APC40 is greater than or equal to the threshold temperature at step S160, the controller 90 calculates for NH generation at step S170₃The rich duration of (c) and the target rich AFR. If the delay time has elapsed after the AFR control is to the rich AFR, the CO begins to escape from the CUC 60. That is, the CUC60 is able to purge CO escaping from the TWC 30 during the delay time, but after the delay time, it cannot adequately purge CO escaping from the TWC 30.

Therefore, if the number of times of entering the rich AFR state is increased while the rich duration kept at the rich AFR is reduced, the CO emission is reduced, and at the same time, the NOx contained in the exhaust gas can be sufficiently purified. For example, if the rich duration for which the AFR remains rich is set to 9 seconds and the number of times to enter the rich AFR is set to 11 times, the TWC 30 generates 0.78g NH in 99 seconds (9 seconds × 11 times)₃And 0.78g NH₃About 2.1g of NOx can be purified. In this case, the amount of CO evolved from CUC60 was about 0.1 g. Therefore, it is desirable to increase the number of entries into the rich AFR while decreasing the rich duration to reduce the amount of CO slip from the CUC60 while generating a sufficient amount of NH in the TWC 30₃. Therefore, the rich duration is calculated such that if the engine is operated at the target rich AFR for the rich duration, the escape amount of CO accumulated downstream of the CUC60 is less than or equal to a predetermined value. Alternatively, the rich duration may be such that when engine 10 is operating at the target rich AFR, it is reached that CO begins to escape from CUC60A period of time. Further, the target rich AFR may be set by one of ordinary skill in the art to improve the performance of the aftertreatment system while reducing fuel consumption. For example, the target rich AFR may be 0.97, i.e., a slightly rich AFR, but is not limited thereto. Furthermore, the CO purification capacity of CUC60 varies with the temperature of CUC 60. Therefore, the rich duration may be calculated based on the target rich AFR and the temperature of the CUC 60.

If the rich duration and the target rich AFR are calculated at step S170, the controller 90 operates the engine 10 at the target rich AFR for the rich duration at step S180. That is, by operating the engine 10 at the target rich AFR for the rich duration, the TWC 30 and the APC40 may generate NH₃While reducing the amount of CO escaping from CUC 60.

After executing step S180, the controller 90 returns to step S130 and operates the engine 10 at the lean AFR. NH even with the engine 10 operating at the target rich AFR for the rich duration₃The formation of NOx may also be insufficient to purge the SCR catalyst 50 (e.g., the amount of NOx flowing into the SCR catalyst 50 is greater than the NH during the rich duration₃The amount of NOx that can be purified is generated). Therefore, the controller 90 operates the engine at the lean AFR for a predetermined time and then executes steps S140 to S180 again. If NH is generated₃Sufficient to purify the NOx flowing into the SCR catalyst 50, the aftertreatment method will terminate at step S150.

Meanwhile, if the temperature of the APC40 is below the threshold temperature at step S160, the controller 90 heats the APC40 before entering the rich AFR state. That is, the controller 90 operates the engine 10 at the stoichiometric AFR (i.e., λ ═ 1) for a first predetermined duration at step S190.

Thereafter, the controller 90 operates the engine 10 at the target lean AFR for a second predetermined duration at step S200, and then returns to step S160 to compare the temperature of the APC40 to the threshold temperature. Wherein the target lean AFR can be arbitrarily set by one of ordinary skill in the art to meet the design intent. In one aspect, the target lean AFR may be between 1.4 and 2.0 based on the detected value of the first oxygen sensor 32.

If the temperature of the APC40 is higher than or equal to the threshold temperature at step S160, the controller 90 executes step S170 and step S180 in this order.

(test method)

The TWC 30, GPF 70, APC40, SCR catalyst 50, and CUC60 are disposed on the exhaust pipe 20 in this order. After that, a four-cylinder lean burn gasoline engine of 2.0L displacement is connected to the exhaust pipe 20, and then aging processing (aging treatment) is performed. The ageing treatment was carried out on the basis of TWC 30 at 1000 ℃ for 50 hours.

The lean AFR (λ ═ 1.8) is maintained at 2000rpm engine speed for 5 minutes to place the entire aftertreatment system in a lean atmosphere and the APC40 temperature is maintained at 365 ℃. Thereafter, the engine 10 is operated at stoichiometric AFR for 12 seconds, and then again at lean AFR (λ ═ 1.8). At this time, the temperature of the TWC 30, the temperature of the APC40, and the concentration of stored NOx that escapes from the APC40 are detected.

FIG. 6 is a graph illustrating the temperature of the TWC, the temperature of the APC, and the concentration of stored NOx escaping from the APC when the engine is operated sequentially at lean AFR, stoichiometric AFR, and lean AFR.

In fig. 6, a thick solid line indicates the temperature of the TWC 30, a thin solid line indicates the temperature of the APC40, and a broken line indicates the concentration of stored NOx that escapes from the APC 40.

If the engine 10 is operating at the stoichiometric AFR for 12 seconds, the temperature of the TWC 30 rises sharply and the temperature of the APC40 rises slowly. Thus, there is a time difference between after the TWC 30 heats up until the APC40 heats up. If the engine 10 is again operated at lean AFR, the temperature of the TWC 30 rises to 700 deg.C and then drops sharply, and the temperature of the APC40 gradually rises in about 20 seconds to about 30 seconds and then drops slowly. When the APC40 temperature reaches 420 deg.C, the TWC 30 temperature is approximately 600 deg.C. In summary, if the engine 10 is operated only at the stoichiometric AFR to heat the APC40 to the target temperature, the temperature of the TWC 30 may rise excessively. Therefore, the temperature of the SCR catalyst 50 also increases, and the NOx purification performance can also be improved. It can also be seen that the APC40 can be heated to a target temperature (e.g., a threshold temperature) even if the engine 10 is operated at a stoichiometric AFR for a first predetermined duration, and then operated at a lean AFR.

Since the APC40 stores NOx contained in the exhaust gas at a lean AFR, if the engine 10 starts operating at a stoichiometric AFR, the amount of stored NOx released from the APC40 may increase. Thereafter, since the amount of NOx stored in the APC40 is reduced, the amount of stored NOx released from the APC40 is also reduced. When the engine 10 is again operated under the lean AFR, the APC40 again stores NOx contained in the exhaust gas, thereby increasing the amount of stored NOx released from the APC 40. Since the amount of stored NOx by the APC40 also correlates with the temperature of the APC40, the amount of stored NOx released from the APC40 increases as the temperature of the APC40 increases.

On the other hand, if engine 10 is operating at stoichiometric AFR, the Oxygen Storage Capacity (OSC) of CUC60 may be consumed. If engine 10 is operated at a rich AFR without restoring the OSC of CUC60, the CO-cleanup capability of CUC60 may decrease rapidly. The OSC of CUC60 is also related to the temperature of CUC 60.

In summary, in view of the temperature of the TWC 30, the temperature of the APC40, the amount of NOx stored in the APC40, and the OSC of the CUC60, the engine 10 may be operated at a lean AFR for a second predetermined duration after operating at the stoichiometric AFR for a first predetermined duration to heat the APC 40.

In one aspect, the first predetermined duration may be set to a value of 5 seconds to 15 seconds. In another aspect, the first predetermined duration may be set to 10 seconds. In other aspects, the first predetermined duration may be determined based on the temperature of the APC40 at a time when it is determined that a transition to the rich AFR is desired.

In one aspect, the second predetermined duration may be set to a value of 10 seconds to 30 seconds. In another aspect, the second predetermined duration may be set to a value of 10 seconds to 20 seconds. In other aspects, the second predetermined duration can be determined based on the first predetermined duration, the target lean AFR, and the temperature of the CUC 60.

FIG. 7 is a graph illustrating the concentration of stored NOx that escapes from the APC as a function of temperature of the APC upon entering the rich AFR, and an increase or maximum concentration of nitrous oxide generation. The graph in FIG. 7 is obtained by controlling the temperature of the APC40 in the following manner: the engine 10 is controlled to operate at an engine speed of 1500rpm for a time at a lean AFR, and then the engine 10 is operated at a rich AFR.

In FIG. 7, the thick solid line represents the concentration of stored NOx that escapes from the APC40, and the thin solid line represents nitrous oxide (N) downstream of the APC40₂O) increase or maximum concentration of production.

As shown in FIG. 7, if the temperature of the APC40 is less than 400 ℃ when entering the rich AFR state, no slip of NOx stored in the APC40 occurs. If the temperature of the APC40 is greater than 400 ℃ upon entering the rich AFR condition, the NOx stored in the APC40 begins to slip. If the temperature of the APC40 is greater than 430 ℃ when entering the rich AFR state, the concentration of stored NOx escaping from the APC40 increases dramatically.

Meanwhile, if the temperature of the APC40 is low upon entering the rich AFR state, N generated in the APC40₂The maximum concentration of O is very high. However, when the temperature of the APC40 increases after entering the rich AFR state, N generated in the APC40₂The maximum concentration of O is significantly reduced. If the temperature of the APC40 is greater than 400 ℃ upon entering the rich AFR state, N generated in the APC40₂The maximum concentration of O is less than 40 ppm.

In summary, to reduce the concentration of stored NOx that escapes from the APC40 and N generated in the APC40₂The increase or maximum concentration of O, the temperature of the APC40 should be controlled to a value of 410 deg.C to 430 deg.C after entering the rich AFR. In one aspect, the threshold temperature of the APC40 may be greater than or equal to 410 ℃ and less than or equal to 430 ℃ when entering the rich AFR. In another aspect, the threshold temperature of the APC40 may be 420 ℃ when entering the rich AFR.

The performance of the post-treatment process according to aspects of the present disclosure will be compared to the performance of comparative examples 1 and 2.

(aspects of the present disclosure)

In an aftertreatment method according to an aspect of the disclosure, the engine 10 is operated at stoichiometric and lean AFR in sequence, and then at rich AFR if a transition to rich AFR is desired. In more detail, the engine 10 is operated at a stoichiometric AFR for a first predetermined duration (e.g., 10 seconds) and then at a target lean AFR (e.g., λ 1.8) for a second predetermined duration (e.g., 20 seconds) to heat the APC40 to 420 ℃. Thereafter, the engine 10 is operated at the target rich AFR (e.g., λ ═ 0.97) for a rich duration.

Comparative example 1

In the aftertreatment method according to comparative example 1, the AFR is converted directly to the rich AFR without passing through the stoichiometric AFR and the lean AFR, in the case where a conversion to the rich AFR is desired. In more detail, the engine 10 is operated at the target rich AFR (e.g., λ ═ 0.97) for the rich duration without heating the APC40 (e.g., the temperature of the APC40 is 365 ℃).

Comparative example 2

In the aftertreatment method according to comparative example 2, the engine 10 is operated at stoichiometric AFR and then at rich AFR if a transition to rich AFR is desired. In more detail, the engine 10 is operated at a stoichiometric AFR to heat the APC40 to 420 ℃; the engine 10 is then operated at the target rich AFR (e.g., λ ═ 0.97) for a rich duration.

Fig. 8 is a graph showing the amount of fuel used to heat the APC and the maximum concentration of nitrous oxide generation entering the rich AFR, the case where the engine is not heated, the case where the APC is heated by operating the engine only at the stoichiometric AFR, and the case where the APC is heated by operating the engine sequentially at the stoichiometric AFR and the lean AFR, respectively.

In FIG. 8, a thick solid line represents the amount of fuel used to heat the APC40, and a thin solid line represents N downstream of the APC40₂Maximum concentration of O formation.

In comparative example 1, since the APC40 is not heated, the amount of fuel used to heat the APC40 is zero. However, since the temperature of the APC40 is low (e.g., 365℃.) upon entering the rich AFR state, N in the APC40₂The maximum concentration of O produced is very high. Therefore, an additional control device/catalytic converter is required to reduce the N generated in the APC40₂O。

In comparative example 2, since the engine 10 is only inThe stoichiometric AFR operation is run to heat the APC40, so the amount of fuel used to heat the APC40 is very large. For example, the amount of fuel for heating the APC40 in the comparative example 2 is more than twice the amount of fuel for heating the APC40 in the aspect of the present invention. Meanwhile, since the temperature of the APC40 in comparative example 2 at the time of entering the rich AFR state is equal to the temperature of the APC40 in aspects of the invention at the time of entering the rich AFR state, N generated in the APC40 in comparative example 2₂The maximum concentration of O is almost the same as in the aspect of the present invention.

In summary, to reduce the amount of fuel used to heat the APC40, and to reduce the N generated in the APC40₂O, the engine 10 may be operated at the rich AFR after being operated at the stoichiometric AFR and the target lean AFR in sequence.

FIG. 9 is a graph showing the accumulated amount of CO slip from the CUC and the increase or maximum concentration of nitrous oxide generation over a predetermined duration of time for entering rich AFR, where the engine is not heating, where the APC is heating by operating the engine only at stoichiometric AFR, and where the APC is heating by operating the engine sequentially at stoichiometric AFR and lean AFR, respectively.

In FIG. 9, the thick solid line represents the accumulated amount of CO slip from the CUC60 when the engine 10 is operating at the rich AFR for 10 seconds, and the thin solid line represents N downstream of the APC40₂An increase or maximum concentration of O production.

In comparative example 1, the OSC of CUC60 was not consumed because engine 10 was not operating at the stoichiometric AFR. Therefore, when engine 10 is operating at a rich AFR for 10 seconds, CO does not escape from CUC 60.

In an aspect of the present disclosure, engine 10 is operated at stoichiometric AFR and the OSC of CUC60 is consumed. However, engine 10 is operated at a lean AFR to recover the OSC of CUC60 before entering the rich AFR state. Thus, when engine 10 is operating at a rich AFR for 10 seconds, CO does not escape from CUC 60.

In comparative example 2, the engine 10 is operated at stoichiometric AFR, thereby consuming the OSC of the CUC60, which then enters a rich AFR state without restoring the OSC of the CUC 60. Therefore, the temperature of the molten metal is controlled,when engine 10 is operating at a rich AFR for 10 seconds, CO will escape from CUC 60. For example, at a rich AFR, 0.9g of CO escapes from the CUC60 within 10 seconds. Therefore, if the rich duration is set such that CO below a predetermined value escapes from CUC60, the rich duration is very short. However, if the rich duration is short, NH is generated in the APC40₃The amount of (c) can also be very small. Additional control means/catalytic converter for reducing CO slip from the CUC60 is required to set the rich duration to generate sufficient NH in the APC40₃。

In summary, the engine 10 may be operated at the rich AFR after operating at the stoichiometric AFR and the target lean AFR in sequence to reduce CO slip from the CUC60 and N generated in the APC40₂O。

While the disclosure has been described in connection with what is presently considered to be practical, it is to be understood that the disclosure is not limited to the disclosed aspects. On the contrary, the disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. An aftertreatment system for a lean burn engine comprising:

an exhaust pipe that is connected to the lean burn engine and through which exhaust gas generated in the lean burn engine flows;

a three-way catalyst that is mounted on the exhaust pipe and purifies hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gas;

an ammonia producing catalyst installed on an exhaust pipe downstream of the three-way catalyst, storing nitrogen oxides at a lean air/fuel ratio, and generating H at a rich air/fuel ratio₂Releasing stored nitrogen oxides and using released nitrogen oxides and generated H₂Generating ammonia;

a selective catalytic reduction catalyst installed on an exhaust pipe downstream of the ammonia producing catalyst to store NH generated in the ammonia producing catalyst₃And using the stored NH₃Reducing nitrogen oxides contained in the exhaust gas;

a CO purification catalyst that is mounted on an exhaust pipe downstream of the selective catalytic reduction catalyst and purifies CO contained in the exhaust gas; and

a controller that detects information on an air/fuel ratio and a temperature of the exhaust gas, and controls the air/fuel ratio of the exhaust gas based on the information on the air/fuel ratio and the temperature of the exhaust gas,

wherein the controller compares the temperature of the ammonia-producing catalyst to a threshold temperature in response to detecting a desired transition to a rich air/fuel ratio and operates the engine at a stoichiometric air/fuel ratio prior to transitioning to a rich air/fuel ratio when the temperature of the ammonia-producing catalyst is less than the threshold temperature.

2. The aftertreatment system of claim 1, wherein the controller operates the engine at a stoichiometric air/fuel ratio for a first predetermined duration.

3. The aftertreatment system of claim 2, wherein the first predetermined duration is determined based on a temperature of the ammonia producing catalyst at a detected time at which a transition to a rich air/fuel ratio is desired.

4. The aftertreatment system of claim 2, wherein the controller operates the engine at a target lean air/fuel ratio for a second predetermined duration after operating the engine at a stoichiometric air/fuel ratio and before transitioning to a rich air/fuel ratio.

5. The aftertreatment system of claim 4, wherein the second predetermined duration is determined based on the first predetermined duration, the target lean air/fuel ratio, and a temperature of the CO purification catalyst.

6. The aftertreatment system of claim 4, wherein the controller operates the engine at a target rich air/fuel ratio for a rich duration after operating the engine at the target lean air/fuel ratio for the second predetermined duration.

7. The aftertreatment system of claim 6, wherein the rich duration is determined based on the target rich air/fuel ratio and a temperature of the CO purification catalyst.

8. The aftertreatment system of claim 6, wherein the rich duration is calculated such that if the engine is operating at the target rich air/fuel ratio for the rich duration, an escape of CO accumulated downstream of the CO purification catalyst during the rich duration is less than or equal to a predetermined value.

9. The aftertreatment system of claim 1, further comprising a particulate filter disposed between the three-way catalyst and the ammonia producing catalyst or between the ammonia producing catalyst and the selective catalytic reduction catalyst,

wherein the particulate filter traps particulate matter in the exhaust gas.

10. An aftertreatment method for controlling an aftertreatment system equipped with a three-way catalyst, an ammonia generating catalyst, a selective catalytic reduction catalyst, and a CO purification catalyst in this order on an exhaust pipe through which exhaust gas flows and which is connected to a lean burn engine, the aftertreatment method comprising:

operating the engine at a lean air/fuel ratio;

calculating NH stored in the selective catalytic reduction catalyst₃The amount of (c);

determining whether a transition to a rich air/fuel ratio is desired;

determining whether a temperature of the ammonia producing catalyst is higher than or equal to a threshold temperature in a case where a shift to a rich air/fuel ratio is desired;

operating the engine at a stoichiometric air/fuel ratio for a first predetermined duration when the temperature of the ammonia-producing catalyst is less than the threshold temperature; and

operating the engine at a target rich air/fuel ratio for a rich duration.

11. The aftertreatment method of claim 10, wherein the first predetermined duration is determined based on a temperature of the ammonia producing catalyst at a determined time at which a transition to a rich air/fuel ratio is desired.

12. The aftertreatment method of claim 10, wherein the rich duration is determined based on the target rich air/fuel ratio and a temperature of the CO purification catalyst.

13. The aftertreatment method according to claim 10, wherein the rich duration is calculated such that an escape amount of CO accumulated downstream of the CO purification catalyst during the rich duration is less than or equal to a predetermined value if the engine is operated at the target rich air/fuel ratio for the rich duration.

14. The post-processing method according to claim 10, further comprising: operating the engine at a target lean air/fuel ratio for a second predetermined duration after operating the engine at a stoichiometric air/fuel ratio for the first predetermined duration and before operating the engine at a rich air/fuel ratio for the rich duration.

15. The aftertreatment method of claim 14, wherein the second predetermined duration is determined based on the first predetermined duration, the target lean air/fuel ratio, and a temperature of the CO purification catalyst.

16. The aftertreatment method of claim 10, wherein the determination of whether a transition to a rich air/fuel ratio is desired includes calculating an amount of nitrogen oxides that will flow into the selective catalytic reduction catalyst, and

wherein NH when stored in the selective catalytic reduction catalyst₃In an amount less than the amount of NH required to purify the nitrogen oxides that will flow into the selective catalytic reduction catalyst₃Is determined to be a desired transition to a rich air/fuel ratio.

17. The aftertreatment method of claim 10, wherein the determination of whether a transition to a rich air/fuel ratio is desired includes storing NH in the selective catalytic reduction catalyst₃With a predetermined NH ratio₃The lower threshold is compared, an

Wherein NH when stored in the selective catalytic reduction catalyst₃Is less than the predetermined NH₃The lower threshold is determined to be a desired transition to a rich air/fuel ratio.